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 R U ready for R-Values & U-Values?

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​​​R U ready for R-Values & U-Values? ​

Between R-values, U-values and λ-values thermal insulation can present an alphabet of confusion. What’s the real difference and why has the λ-value ‘won’ when it comes to pipe insulation?​


​​Thermal insulation prevents heat transfer by restricting the amount of conduction that’s able to take place through the material. Thermal conductivity is an inherent property of all materials and is denoted by the greek symbol ‘λ’ (pronounced lambda).

A failing of the λ-value is that although it makes it easy for a user to compare the likely performance of two or more different insulation materials it doesn’t take the insulation thickness into account. This means that when presented with two insulation materials that have different λ-values and different thicknesses it can be hard to tell which one will provide more effective insulation.​


The R-value solves this problem by incorporating the thickness and creating a measure of thermal resistance. For flat sheet the R-value is literally just the insulation thickness divided by the insulation thickness divided by the λ-value.

​​R-values for pipework are slightly more complex and determined by the equation:

R-value equation

R-values are a measure of thermal resistance and so, for thermal insulation, a high R-value is better than a low R-value.​


​​Very closely related to the R-value is the U-value. Just the reciprocal of the thermal resistance (or R-value), U-values have many of the same properties. The main difference between R-values and U-values is that when it comes to U-values a low value is better than a high value.

Although not commonly used for mechanical insulation the U-value is the most popular measure of structural insulation performance.​

Why the λ-value wins
Best performanceLowest valueHighest valueLowest value
Thickness dependentNoYesYes
Temperature dependentYesYesYes
Pipe size dependentNoYesYes

​​It’s easy to see why the λ-value wins by looking at the table above.

​​Both the R-value and U value change as the pipe size changes. This means that the same thickness of insulation will have a different R-value and U-value for each size of pipe. With Kaiflex produced in tubes to fit a wide range of different pipe sizes we would have to publish a vast table of R-values and U-values instead of a single λ-value!

Structural insulation materials are supplied in flat sheets or slabs – making R-values and U-values more manageable.​

From the hottest pipes to the coldest

​​The temperature dependence of λ-values, R-values and U-values would also increase the scale of the task by a further order of magnitude. Temperatures inside and outside of a building are fairly constant – never varying by more than around 30-40°C. As a result the λ-values, R-values and U-values of a structural insulation material won’t change much throughout the year and it’s reasonable to publish values suitable for a single average temperature.

​​Mechanical insulation has no such luxury!

​​Kaiflex insulation can be used on pipes as hot as 150°C and as low as -200°C. Across such a wide range of temperatures the thermal conductivity can experience substantial changes. To reflect this using U-values and R-values just isn’t realistic – we’d need to publish a vast range of tables!

This, perhaps more than anything else explains why, when it comes to mechanical insulation, λ-values won out definitively and why you won’t see U-values and R-values stated on our technical datasheets.​

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Between R-values, U-values and λ-values thermal insulation can present an alphabet of confusion. What’s the real difference and why has the λ-value ‘won’ when it comes to pipe insulation?


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